1. Field of the Invention
The present invention relates to a radiation imaging apparatus, radiation imaging method, and storage medium.
2. Description of the Related Art
Recently, with advances in digital technology, digital processing is generally performed on, for example, images obtained by medical X-ray radioscopy. In place of conventional X-ray imaging using films for X-ray diagnosis, a two-dimensional X-ray sensor which can output X-ray images as digital data has also been developed. Digital image processing such as halftone processing is indispensable to a radiation imaging apparatus such as an X-ray radioscopy apparatus using a two-dimensional sensor.
X-ray radioscopy performs radiation exposure control (AEC=Auto Exposure Control) to properly control the amount of X-rays for irradiation by detecting the amount of X-rays transmitted through an object. This radiation exposure control, first of all, acquires feature amounts such as an average value of an X-ray radioscopic image. This X-ray radioscopic image is obtained by X-rays having a pulse-like waveform which are emitted from an X-ray generating unit. This technique then controls X-ray irradiation conditions (for example, the tube voltage, tube current, and X-ray pulse width set for the X-ray generating unit) so as to set a desired exposure based on the comparison between the levels of the feature amounts and reference values.
The purpose of image processing and radiation exposure control in an X-ray radioscopy apparatus is to properly display a region of interest corresponding to an anatomical structure of the human body which is a most important image region in terms of diagnosis.
In image processing and radiation exposure control in an X-ray radioscopy apparatus, a region of interest is extracted from a captured image. Feature amounts used for image processing or radiation exposure control are then calculated from the extracted region of interest. Regions of interest vary depending on imaging target regions and imaging purposes. In radioscopy of the stomach with the use of barium, a region of interest is set on the stomach wall to detect a polyp on the stomach wall. When capturing a moving image of a chest region, a lung field region is set as a region of interest. In cardiac catheterization, a region including the distal end of a catheter and its peripheral region is set as a region of interest.
A region located outside an exposure field when the exposure field is narrowed by a collimator and a so-called direct irradiated region where X-rays directly strike a sensor without passing through an object adversely affect proper region feature calculation. In addition, including a region greatly different in X-ray absorptance from a region of interest, such as a metal piece, in the overall region of interest will adversely affect proper region feature calculation. Such a region therefore should be excluded from the region of interest.
As a conventional method of extracting a region of interest from a given image, there is available threshold processing which sets a threshold for discriminating a region of interest from other regions and extracts a region of interest based on the threshold. In addition, for example, edge extraction processing or the like which extracts the contour shape of an object based on the density distribution shape of an image has been widely used.
For example, Japanese Patent Laid-Open No. 2000-10840 discloses a technique of obtaining the density histogram of an object area in a radiographic image and performing halftone correction processing and dynamic range compression processing for the radiographic image based on the feature amount of the image which is calculated from the density histogram. It is possible to stably extract a feature amount in an object area in an image by extracting image component information corresponding to a bone or soft tissue of an object by using direct irradiated region removal processing and a histogram shape. Even if, for example, the maximum pixel density value in an object area in a radiographic image is smaller than a predetermined pixel density value, this technique allows effective image processing.
In addition, Japanese Patent Laid-Open No. 2005-218581 discloses a technique of extracting an exposure field region for the optimization of image processing parameters. This technique calculates an exposure field candidate region by scoring exposure field likelihoods from a target pixel and its neighboring pixel pattern so as to cope with various collimator shapes such as circular and polygonal shapes. The technique then determines the shape of an exposure field candidate region by obtaining a shape feature amount such as a degree of circularity. The technique extracts an exposure field by an exposure field recognition algorithm specialized to a determined shape. As an algorithm specialized to a determined shape, linear detection processing such as Hough transformation is used for polygonal shapes. Template matching and the like are used for circular shapes. These kinds of processing improve accuracy.
Japanese Patent Laid-Open No. 2003-250789 discloses a technique of extracting a region of interest used for region feature calculation to properly execute at least one of the processes including radiation exposure control and image density conversion in radioscopic imaging that generates images at a relatively low rate of 3 to 5 frames per sec. First, this technique projects (accumulates) image data on an image in the vertical and horizontal directions within a rectangular exposure field region. The technique then generates a one-dimensional array in each direction and executes second derivative computation for the array. The technique then extracts positions at which maximum values are obtained as circumscribed lines (boundary lines) of an exposure field in the respective directions. The technique executes region-of-interest extraction processing for the extracted exposure field region. The technique executes region-of-interest extraction processing by selectively executing an extraction algorithm for each region of interest based on imaging target region information or order (request) information. Region-of-interest extraction techniques include a technique of setting a region of interest as a predetermined region, a technique of detecting a stomach wall (region of interest) as a region around the contour of a barium block, and a technique of detecting a lung field region (region of interest) at the time of capturing a moving image of a chest region. With regards to these techniques, there are disclosed algorithms using image histogram analysis, morphology computation, logical operation using binary images, and the like.
In general, however, an algorithm for extracting a region of interest and calculating a feature amount from the region of interest is complex. It is difficult, especially in an apparatus which processes a large amount of data as in X-ray radioscopic imaging, to accurately extract feature amounts at a required high frame rate (25 fps to 30 fps) in the interval between X-ray irradiation and display. The above conventional techniques are designed to extract feature amounts from images obtained by still image capturing or radioscopic imaging at a relatively low frame rate.
When a conventional technique is applied to radioscopic imaging at a high frame rate, since the feature amount extraction algorithm cannot follow the high frame rate, there is a delay between the start of processing and the calculation of a feature amount. As a result, the information of a frame acquired after the start of feature amount extraction processing is not reflected in an extracted feature amount. This makes it impossible to apply image processing and X-ray control based on an optimal feature amount. Furthermore, when an object makes a large movement, feature amount extraction processing itself sometimes becomes useless. Assume that feature amount extraction processing is started before the movement of an object, and a feature amount is extracted after the movement of the object. In this case, it is impossible to use this feature amount for image processing and X-ray control.
In consideration of the above problem, the present invention provides a radiation imaging apparatus which performs proper feature amount extraction with the information of a new frame being also reflected in radiographic moving image capturing.